Present day video encoders implementing video compression standards coming from the MPEG organization (MPEG-1, MPEG-2, MPEG-4, . . . ) or from the International Telecommunications Union ITU-T (H.261, . . . , H.264/AVC), are used to encode a sequence of images to deliver an encoded sequence, compressed in relation to the source image sequence.
To this end, these encoders use an encoding scheme using a prediction between the images of the sequence in order to obtain a major level of performance in compression.
It may be recalled that the images of a sequence are conventionally subdivided into slices which are then subdivided into macroblocks. Each macroblock is then subdivided into blocks of pixels where a block of pixels corresponds to a grouping of adjacent pixels. A block of pixels of an image of the sequence can then be encoded by temporal prediction from one or more blocks of one or more other images of the sequence. The term used then is “inter” block. A block of pixels can also be encoded by spatial prediction from one or more blocks of the image to be encoded. The term used then is “intra” block.
During the encoding of a block, it is specified whether or not this block is encoded by using information coming from other images of the sequence known as reference images. These reference images are images that have been encoded/decoded previously. A reference block corresponds to a grouping of pixels of a reference image.
More specifically, the encoding of an inter block runs in two steps:                a prediction of the block to be encoded is first of all made by using one or more reference blocks and a motion compensation mechanism to take account of an apparent motion or again to achieve an efficient prediction;        a prediction error residue is then computed, by determining the difference between the block to be encoded and the prediction.        
The difference, or residue, between the current block and the predicted block is then encoded and transmitted (and/or stored depending on the applications). At the time of decoding, the received difference is added to the prediction to reconstruct the block.
Classically, in video compression standards, prediction by motion compensation of an inter block is achieved by means of a translation-type motion compensation. Thus, if v denotes the motion vector considered, and R is the reference image considered, the prediction P at each point or pixel x of the block is defined by: P(x)=R(x+v).
One drawback of this motion compensation technique is that it cannot be used to take account of natural motions of a rotation, zoom, shearing or other type.
Other motion compensation variants are then proposed to make the prediction.
For example, P. Ishwar and P. Moulin in “On Spatial Adaptation of Motion Field Smoothness in Video Coding”, have proposed a block-based motion compensation technique known as OBMC (Overlapped Block Motion Compensation) which consists in considering several values of motion vectors to set up a value of prediction of a pixel x of a block to be encoded:
      P    ⁡          (      x      )        =            ∑      i        ⁢                            w          i                ⁡                  (          x          )                    ⁢              R        ⁡                  (                      x            +                          v              i                                )                    where: the vectors vi correspond to i motion vectors,                the values wi(x) correspond to the weighting values between the different predictions obtained by the different motion vectors.        
The weighting values wi(x) are generally dependent on the position of the point x considered in the block. Typically, the weights wi(x) used are the same for all the blocks.
Although this technique can be used to take account of a large family of natural motions, the prediction obtained is of average quality. The volume of information of prediction residue to be transmitted is therefore greater and this transmission of additional information entails penalties in terms of compression performance.
There is therefore a need for new image encoding/decoding techniques, having low complexity while at the same time providing high-quality prediction, to at least partially mitigate the drawbacks of the prior art.